As feature sizes on semiconductor devices shrink, increased current density is required in the conductive lines. However, use of a high current density in these devices is limited because of electromigration in conductors, such as copper. Electromigration is the transport of material in a conductor caused by gradual movement of atoms or ions due to the momentum transfer between conducting electrons and the atoms or ions. Momentum from a moving electron may transfer to a nearby atom or ion and cause it to move from its original position. Over time, a large number of these atoms or ions may be moved, decreasing the reliability of electrical circuits.
Electromigration may cause gaps, breaks, increased line resistance, and excess heating in a semiconductor device. A gap or break in the conducting material or an unintended electrical connection may form when a number of atoms or ions move. Formation of such a gap or break may prevent the flow of electrons. Unintended connections may cause the device to malfunction.
Furthermore, as the conductive line-widths decrease, line resistance increases. Line resistance is the resistance across the length of a conductive layer having, for example, a rectangular cross-section. This may be represented by the equation:R=ρ(L/wt)In this equation, R is the line resistance in ohms per nm, ρ is the film resistivity in ohms, L is the length of the conductor in nm, w is the width of the conductor in nm, and t is the thickness of the conductor in nm. High current density and resistance at a particular point in the conductor also may generate heat that may cause the breakdown of the conductor. Any imperfections in the crystal lattice may cause electrons flowing through the conductor to scatter rather than pass through the crystal lattice without collisions. Scattering of electrons will cause atoms in the crystal lattice to vibrate farther from an ideal position in the crystal lattice and, consequently, will increase resistance in the conductor. This increased resistance may lead to joule heating, or heat released when a current passes through a conductor, and undesirable losses in the semiconductor device.
Even grains within the conductive layers of a semiconductor structure may begin to move due to electromigration. Grain boundaries lack the symmetry and uniformity of a normal crystal lattice, causing momentum from the electrons to be transferred to the atoms or ions in the crystal lattice more strongly. Metal ions near the grain boundary are more weakly bonded than in the normal crystal lattice, so atoms may become separated and be transported in the direction of the current. The atoms may tend to move along the grain boundary. Such movement may change the grains in the semiconductor structure.
Electromigration may decrease the reliability of circuits or devices, such as a complimentary metal oxide semiconductor (CMOS). As the structures or feature sizes of a semiconductor device shrink, the effects caused by electromigration increase because power density and current density will increase. Furthermore, as semiconductor devices or integrated circuits (ICs) become more complex, individual parts or components must become more reliable for the entire semiconductor device or IC to function properly. Electromigration can lead to failures of the individual parts or components, reducing the lifetime of the semiconductor device or IC. This is one reason it is important to control or reduce electromigration in a conductor.
Many techniques have been employed to improve copper electromigration. One provides capping layers at the top of a metal line. The capping layers may include CoWP/CoWB or SiCN. Another method is alloying the copper line with CuAl or CuMn. Lastly, ion and gas cluster implantation of a single species into the copper layer has been performed to modify the surface structure of the copper layer to improve adhesion or electric properties. None of these methods, however, reduce line resistance and prevent electromigration at the same time. Accordingly, there is a need for improved methods to reduce electromigration in a conductor, and, more specifically, to reduce electromigration while not substantially increasing the line resistance.